The present invention relates to a memory device utilizing variations in resistance value which can be used not only as an external memory device for a computer or the like but also as an internal memory device incorporated in a ULSI and to a method of manufacturing the same.
With recent remarkable improvements in the performance of a processor or the like composed of a ULSI to be contained in a computer, an external memory device having a high capacity has been in increasing demand. To compose the high-capacity external memory device, such devices as a magnetic disk, a CD-ROM, and an IC memory device using a silicon semiconductor have commonly been used.
Among them, a non-volatile memory using fuses as disclosed in U.S. Pat. No. 5,278,784 has particularly been known. FIG. 13 shows a memory cell array disclosed in the above publication, which consists of a large number of memory cells arranged to form a matrix. The individual memory cells are connected by a word line 100 and bit lines 101a to 101c along rows and a column. In the drawing are shown: select transistors 106a to 106c which have their gates connected to the above word line 100; auxiliary transistors 105a and 105b which connect contiguous memory cells; a common source line 103 connected to each of the sources of the above select transistors 106a to 106c. Between the memory cells and the bit lines 101a to 101c are interposed respective fuses 104a to 104c each of which functions as a non-volatile memory cell. Any one of the fuses 104a to 104c is melted down by allowing a large current to flow therethrough. If the disconnected state and connected state of the fuses 104a to 104c are defined as "1" and "0", respectively, the fuses 104a to 104c can be utilized as non-volatile memory (ROM) cells.
However, the conventional magnetic disk, CD-ROM, and non-volatile memory such as a ROM using fuses have the following drawbacks.
Although the magnetic disk or CD-ROM can be designed to have a larger capacity, access time is also increased. Consequently, even if the performance of a computer is to be increased by increasing the capacity of such a memory, the computer cannot fully achieve its intrinsic performance. Consequently, an external memory device has presented an obstacle to improved performance of the computer.
On the other hand, although an IC memory device and the foregoing ROM using fuses can be accessed at substantially the same speed as a ULSI, chip size is also increased with increases in capacity, so that manufacturing cost is increased, resulting in extremely high price.
Moreover, since it is structurally impossible to rewrite to these non-volatile memories, they only have limited applications. For example, a non-volatile memory cannot be used as a RAM.